In recent years, displays in image display devices are being shifted from conventional cathode-ray tube displays to thin display panels, such as liquid crystal panels and plasma display panels. With the thin displays, thicknesses of the image display devices can be decreased. Liquid crystal panels used for the liquid crystal display device do not emit light. Therefore, liquid crystal display devices including liquid crystal panels require backlight devices. The backlight devices are classified broadly into a direct type and an edge-light type based on mechanisms. The edge-light type backlight device includes a light source arranged on an edge portion, a light guide plate that guides light from the light source, and optical member that changes optical properties of light from the light guide plate and provides even planar light to the liquid crystal panel. A backlight device disclosed in Patent Document 1 is known as an example of the kind. In the configuration of Patent Document 1, the light exit surface or the opposing surface of the light guide extends in a direction facing the light entrance surface and includes multiple convex and concave portions in the light guide plate. The convex and concave portions include a flat surface on a bottom portion of a concave portion or on a top portion of a convex portion.